1. Field of the Invention
The present invention relates generally to the soldering of metallic surfaces. More specifically, the present invention relates to a coating or film which protects the metallic surface from contamination while being stored and provides at the same time for proper fluxing of the surface to enhance solderability at the solder temperature.
2. Description of Related Art
The manufacture of active circuit boards requires various soldering operations which must be carried out either manually, by some automated means, or both. A prerequisite for successful solder joints is proper wetting of all mating parts, component leads, or any other items to be fastened by the solder.
The solder operation requires proper fluxing agents, for without them, reliable electrical and/or mechanical connection may not be made even on apparently clean surfaces. Circuit boards and components, as received, are not necessarily clean and may carry various kinds of surface contaminants. Subsequent storage generally adds to these contaminants and does not improve upon solderability. As a result, some components may be adequately soldered when exposed to activated fluxing agents prior to soldering while some surfaces do not solder well even after treatment with activated flux.
For a reliable and repeatable soldering operation, especially in automated production lines, the following requirements are absolutely essential: (1) clean and solderable surfaces, (2) suitable solder alloys, (3) proper fluxing agents, (4) reproducible heat inputs and time of application, and (5) ease of flux/residue removal. Adequate solutions to all of these requirements have been met, except for the first requirement of a clean and solderable surface. For the most part, as long as the mating metallic surfaces are kept clean, reliable solder joints may be made. However, if the surface is not sufficiently clean, unreliable solder joints result. Further, currently used methods for cleaning surfaces to be soldered frequently use solvents such as halogenated hydrocarbons which have a negative impact on the environment.
The problems with imperfect solder joints or connections are generally due to contaminated surfaces which represent low energy sites. The adhesive forces acting on the metallic-solder interface must be larger than the cohesive forces of the corresponding solder. Without this, the solder will not wet the metal surface properly and in the end, a cosmetically acceptable but defective solder joint results. In order to prevent this problem, it is essential that the metallic surfaces be kept clean in the initial phase of the soldering operation. In addition, it is essential that high free-energy surfaces be present in the initial phase of the soldering operation so that subsequent chemical reaction will form a strong intermetallic bond between the metal and solder.
Examination of properly soldered joints shows that the solder has undergone a structural change as it bonds with the metal surfaces. There is a well defined intermetallic layer between the metallic surface and solder. For example, when using a lead-tin solder on a copper surface, this layer is a finely crystalline layer which comprises predominantly Cu.sub.6 Sn.sub.5. Simultaneously with the growth of the inter-metallic layer, wetting is improved and the joining of the solder proceeds at a much faster rate. The initially fine crystalline intermetallic layer may become coarsely crystalline after a prolonged heat treatment, especially during rework, and may lead, in the end, to premature bond failure.
The mere introduction of molten solder into a narrow space between two metal surfaces does not necessarily result in the wetting of either surface. The ensuing alloying or formation of the intermetallic layer, which is a prerequisite of proper adhesion is often negated by surface contaminants. Molten solder, by itself, will not dissolve or displace the contaminants. It therefore is desirable to develop effective methods and means for the preservation and protection of high-energy metallic surfaces prior to and during the soldering operation. In addition, it is desirable to convert the low-energy (contaminated) surfaces to high-energy surfaces wherever possible to enhance solderability.